Multi-tiered memory with different metadata levels

ABSTRACT

Method and apparatus for managing data in a memory. In accordance with some embodiments, a data object is stored in a first non-volatile tier of a multi-tier memory structure. A metadata unit is generated to describe the data object, the metadata unit having a selected granularity. The metadata unit is stored in a different, second non-volatile tier of the multi-tier memory structure responsive to the selected granularity.

SUMMARY

Various embodiments of the present disclosure are generally directed tomanaging data in a data storage device.

In accordance with some embodiments, a data object is stored in a firstnon-volatile tier of a multi-tier memory structure. A metadata unit isgenerated to describe the data object, the metadata unit having aselected granularity. The metadata unit is stored in a different, secondnon-volatile tier of the multi-tier memory structure responsive to theselected granularity.

These and other features and aspects which characterize variousembodiments of the present disclosure can be understood in view of thefollowing detailed discussion and the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 provides is a functional block representation of a data storagedevice in accordance with various embodiments of the present disclosure.

FIG. 2 illustrates exemplary formats for a data object and acorresponding metadata unit used to describe the data object.

FIG. 3 depicts portions of the device of FIG. 1 in greater detail.

FIG. 4A provides an illustrative format for a first data object fromFIG. 3.

FIG. 4B is an illustrative format for a second data object from FIG. 3.

FIG. 5 depicts aspects of the data object storage manager of FIG. 3 ingreater detail.

FIG. 6 shows aspects of the metadata storage manager of FIG. 3 ingreater detail.

FIG. 7 represents a sequence of steps carried out in accordance withsome embodiments to service read requests from a requestor.

FIG. 8 represents a sequence of steps carried out in accordance withsome embodiments to service write requests from a requestor.

FIG. 9 depicts a metadata granularity adjustment module of the device ofFIG. 1 that operates in accordance with some embodiments.

DETAILED DESCRIPTION

The present disclosure generally relates to the management of data in adata storage device.

Data storage devices generally operate to store blocks of data inmemory. The devices can employ data management systems to track thephysical locations of the blocks so that the blocks can be subsequentlyretrieved responsive to a read request for the stored data. The devicemay be provided with a hierarchical (multi-tiered) memory structure withdifferent types of memory to accommodate data having differentattributes and workloads. The various memory tiers may be erasable orrewriteable.

Erasable memories (e.g, flash memory, write many optical disc media,etc.) are non-volatile memories that usually require an erasureoperation before new data can be written to a given memory location. Itis thus common in an erasable memory to write an updated data set to anew, different location and to mark the previously stored version of thedata as stale.

Rewriteable memories (e.g., dynamic random access memory (DRAM),resistive random access memory (RRAM), magnetic disc media, etc.) may bevolatile or non-volatile, and are rewriteable so that an updated dataset can be overwritten onto an existing, older version of the data in agiven location without the need for an intervening erasure operation.

Metadata can he generated and maintained to track the locations andstatus of the stored data. The metadata tracks the relationship betweenlogical elements (such as logical block addresses, LBAs) stored in thememory space and physical locations (such as physical block addresses,PBAs) of the memory space. The metadata can also include stateinformation associated with the stored data and/or the memory locationof the stored data, such as the total number of accumulatedwrites/erasures/reads, aging, drift parametrics, estimated or measuredwear, etc.

Data management systems often extend considerable effort to maintain themetatadata in an up-to-date and accurate condition, since the metadataare used to retrieve data responsive to host data requests. Whileexisting metadata management schemes are operable, there is a need forimprovements in the manner in which metadata are generated, stored andmanaged.

Various embodiments of the present disclosure provide a multi-tieredmemory structure adapted to store data objects and associated metadata.As explained below, a data object is stored in a first non-volatilememory tier of the multi-tiered memory structure. A metadata unit isgenerated to describe the data object, and the metadata unit is storedin a different, second non-volatile memory tier of the multi-tieredmemory structure. The metadata unit has a selected granularityresponsive to usage characteristics of the data object.

Mechanisms are provided to select the respective tiers in which the dataobjects and metadata are stored based on a number of attributes of theuser data, memory and/or workload. In this way, the size and the updatefrequency of the metadata unit can be paired to the type of media towhich the metadata unit is stored.

These and other features of various embodiments can be understoodbeginning with a review of FIG. 1, which provides a functional blockrepresentation of a data storage device 100. The device 100 includes acontroller 102 and a multi-tiered memory structure 104. The controller102 provides top level control of the device 100, and the memorystructure 104 stores and retrieves user data from/to a requestor entity,such as an external host device (not separately shown).

The memory structure 104 includes a number of memory tiers 106, 108 and110 denoted as MEM 1-3. The number and types of memory in the varioustiers can vary as desired. Generally, the higher tiers in the memorystructure 104 may be constructed of smaller and/or faster memory and thelower tiers in the memory structure may be constructed of larger and/orslower memory.

For purposes of providing one concrete example, the system 100 iscontemplated as a flash memory-based storage device, such as a solidstate drive (SSD), a portable thumb drive, a memory stick, a memorycard, a hybrid storage device, etc. so that at least one of the lowermemory tiers provides a main store that utilizes erasable flash memory.At least one of the higher memory tiers provides rewriteablenon-volatile memory such as resistive random access memory (RRAM), phasechange random access memory (PCRAM), spin-torque transfer random accessmemory (STRAM), etc. This is merely illustrative and not limiting. Otherlevels may be incorporated into the memory structure, such as volatileor non-volatile cache levels, buffers, etc.

FIG. 2 illustrates exemplary formats for a data object 112 andassociated metadata unit 114 that can be used by the device 100 ofFIG. 1. Other formats may be used. The data object 112 is managed as anaddressable unit and may be formed from one or more data blocks suppliedby the requestor (host). The metadata unit 114 provides controlinformation to enable the device 100 to locate and retrieve thepreviously stored data object 112. The metadata unit 114 will tend to besignificantly smaller (in terms of total number of bits) than the dataobject 112 to maximize data storage capacity of the device 100.

The data object 112 includes header information 116, user data 118, oneor more hash values 120 and error correction code (ECC) information 122.The header information 116 may be the LBA value(s) associated with theuser data 118 or other useful identifier information. The user data 118comprise the actual substantive content supplied by the requestor forstorage by the device 100.

The hash value 120 can be generated from the user data 118 using asuitable hash function, such as a Sha hash, and can be used to reducewrite amplification by comparing the hash value of a previously storedLBA (or range of LBAs) to the hash value for a newer version of the sameLBA (or range of LBAs). If the hash values match, the newer version maynot need to be stored to the structure 104 as this may represent aduplicate set of the same user data.

The ECC information 122 can take a variety of suitable forms such asoutercode, parity values, IOEDC values, etc., and is used to detect andcorrect up to a selected number of errors in the data object during readback of the data.

The metadata unit 114 may include a variety of different types ofcontrol data such as data object (DO) address information 124, DOattribute information 126, memory (MEM) attribute information 128, oneor more forward pointers 130 and a status value 132. Other metadata unitformats can be used. The address information 124 identifies the physicaladdress of the data object 112, and may provide logical to physicaladdress conversion information as well. The physical address willinclude which tier (e.g., MEM 1-3 in FIG. 1) stores the data object 112,as well as the physical location within the associated tier at which thedata object 112 is stored using appropriate address identifiers such asrow (cache line), die, array, plane, erasure block, page, bit offset,and/or other address values.

The DO attribute information 126 identifies attributes associated withthe data object 112, such as status, revision level, timestamp data,workload indicators, etc. The memory attribute information 128 providesparametric attributes associated with the physical location at which thedata object 112 is stored. Examples include total number ofwrites/erasures, total number of reads, estimated or measured weareffects, charge or resistance drift parameters, bit error rate (BER)measurements, aging, etc. These respective sets of attributes 128, 130can be maintained by the controller and/or updated based on previousmetadata entries.

The forward pointers 130 are used to enable searching for the mostcurrent version of the data object 112. The status value(s) 132 indicatethe current status of the associated data object (e.g., stale, valid,etc.).

FIG. 3 is a functional block representation of portions of the device100 in accordance with some embodiments. Each time the device 100receives one or more data blocks (LBAs) from the requestor for storageto the memory structure 104, a data object (DO) storage manager 140generates a data object such as 112 in FIG. 2, and a metadata (MD)storage manager 142 generates a corresponding metadata unit such as 114.The respective storage managers 140, 142 evaluate a number of attributesto form the respective data object and metadata unit, and determine anappropriate memory tier in the structure 104 for the storage of each.

The data object (DO) and the metadata unit (MD) may be stored in thesame tier, but it is contemplated that different tiers will often beused for these respective types of data based on the characteristics ofthe user data and the memory. In some cases, the MD unit will be storedin a lower tier as compared to the DO. In other cases, the MD unit willbe stored in a higher tier as compared to the DO. The granularity of theMD unit can be selected in response to characteristics of the DO, andthe memory tier for the MD unit can in turn be selected based on theselected MD granularity.

The multi-tiered memory structure 104 is shown in FIG. 3 to include anumber of exemplary tiers including an NV-RAM module 144, an RRAM module146, a PCRAM module 148, an STRAM module 150, a flash module 152 and adisc module 154. These are merely exemplary as any number of differenttypes and arrangements of memory modules can be used in various tiers asdesired.

The NV-RAM 144 comprises volatile SRAM or DRAM with a dedicated batterybackup or other mechanism to maintain the stored data in a non-volatilestate. The RRAM 146 comprises an array of erasable non-volatile memorycells that store data in relation to different programmed electricalresistance levels responsive to the migration of ions across aninterface. The PCRAM 148 comprises an array of phase change memory cellsthat exhibit different programmed resistances based on changes in phaseof a material between crystalline (low resistance) and amorphous (highresistance).

The STRAM 150 comprises an array of memory cells each having at leastone magnetic tunneling junction made up of a reference layer of materialwith a fixed magnetic orientation and a free layer having a variablemagnetic orientation. The effective electrical resistance, and hence,the programmed state, of each MTJ can be established in relation to theprogrammed magnetic orientation of the free layer.

The flash memory 150 comprises an array of flash memory cells whichstore data in relation to an amount of accumulated charge on a floatinggate structure. Unlike the NV-RAM 144, RRAM 146, PCRAM 148 and STRAM150, which are all contemplated as comprising rewriteable non-volatilememory tiers, the flash memory 152 is erasable so that an erasureoperation is generally required before a unit of data (e.g., an erasureblock or larger garbage collection unit) can be reconditioned andreturned to an allocation pool to receive newly written data. The flashmemory cells can be configured as single level cells (SLCs) ormulti-level cells (MLCs) so that each memory cell stores a single bit(in the case of an SLC) or multiple bits (in the case of an MLC).

The disc memory 154 may be magnetic rotatable media such as a hard discdrive (HDD) or similar storage device. Other sequences, combinations andnumbers of tiers can be utilized as desired, including other forms ofsolid-state and/or disc memory, remote server memory, volatile andnon-volatile buffer layers, processor caches, intermediate caches, etc.

It is contemplated that each tier will have its own associated memorystorage attributes (e.g., capacity, data unit size, I/O data transferrates, endurance, etc.). The highest order tier (e.g., the NV-RAM 144 inFIG. 3) will tend to have the fastest I/O data transfer rate performance(or other suitable performance metric) and the lowest order tier (e.g.,the disc 154) will tend to have the slowest performance. Each of theremaining tiers will have intermediate performance characteristics in aroughly sequential fashion.

As shown by FIG. 3, the DO storage manager 140 generates two successivedata objects in response to the receipt of different sets of data blocksfrom the requestor, a first data object (OB1) and a second data object(OB2). In the present example, the storage manager 140 directs thestorage of the DO1 data in the flash memory 152, and directs the storageof the DO2 data in the RRAM module 146. The MD storage manager 142generates two corresponding metadata units MD1 and MD2 for the first andsecond data objects OB1 and OB2. The MD1 unit is stored in the PCRAM148, and the MD2 unit is stored in the STRAM 150. It will be appreciatedthat in each case, the specific memory tier for the storage of the dataobjects DO1 and DO2, and the corresponding metadata units MD1 and MD2,can be tailored to accommodate a number of factors as discussed infurther detail below.

Although not limiting, it is contemplated that the first data object DO1from FIG. 3 represents a relatively large amount of user data comprisingmultiple LBAs 156 (from LBA X to LBA N) as depicted in FIG. 4A. Thecorresponding metadata unit MD1 describes these LBAs and treats theoverall data object DO1 as a single unit. As shown by FIG. 4B, thesecond data object DO2 from FIG. 3 comprises a single LBA 156 (LBA Y).The corresponding metadata unit MD2 describes the storage of the seconddata object DO2 with a smaller granularity than that of MD1. It will beappreciated that, in terms of overall size, the first data object DO1will tend to be significantly larger than the second data object DO2,but accounting for the level of detail provided therein, the actual sizeof MD2 may be larger than, smaller than, or essentially the same sizeas, MD1.

FIG. 5 depicts the DO storage manager 140 from FIG. 3 in accordance withsome embodiments. A data object (DO) analysis engine 160 receives thedata block(s) (LBAs 156) from the requestor as well as existing metadata(MD) stored in the device 100 associated with prior version(s) of thedata blocks, if such have been previously stored to the memory structure104. Memory tier attribute data maintained in a database 162 may beutilized by the engine 160 as well. The engine 160 analyzes the datablock(s) to determine a suitable format and location for the dataobject. The data object is generated by a DO generator 164 using thecontent of the data block(s) as well as various data-related attributesassociated with the data object. A tier selection module 166 selects theappropriate memory tier of the memory structure 104 in which to storethe generated data object.

The arrangement of the data object, including overall data object size,may be matched to the selected memory tier; for example, page level datasets may be used for storage to the flash memory 152 and LBA sized datasets may be used for the RRAM, PCRAM and STRAM memories 146, 148 and150. Other sizes can be used. The unit size of the data object may ormay not correspond to the unit size utilized at the requestor level. Forexample, the requestor may transfer blocks of user data of nominally 512bytes in size. The data objects may have this same user data capacity,or may have some larger or smaller amounts of user data, includingamounts that are non-integer multiples of the requestor block size. Theoutput DO storage location from the DO tier selection module 166 isprovided as an input to the memory module 104 to direct the storage ofthe data object at the designated physical address in the selectedmemory tier.

Referring again to FIG. 3, the decision to store the DO2 data objectdata in the RRAM module 146 may be based on a predicted or observedhigher write priority as compared to the DO1 data object which wasstored to the flash memory 152. Other DO attributes may influence theselection of the appropriate tier for the storage of the data objects,including read priority (e.g., frequent read requests), requestor levelstatus, spatial or temporal locality to other data blocks, etc.

FIG. 6 depicts portions of the metadata (MD) storage manager 142 fromFIG. 3 in accordance with some embodiments. An MD analysis engine 170uses a number of factors such as the DO attributes, the DO storagelocation, the existing MD (if available) and memory tier informationfrom the database 162 to select a format, granularity and storagelocation for the metadata unit 114. An MD generator 172 generates themetadata unit and a tier selection module 174 selects an appropriatetier level for the metadata. In some cases, multiple data objects may begrouped together and described by a single metadata unit.

As before, the MD tier selection module 174 outputs an MD storagelocation value that directs the memory structure 104 to store themetadata unit at the appropriate physical location in the selectedmemory tier. A top level MD data structure such as MD table 176, whichmay be maintained in a separate memory location or distributed throughthe memory structure 104, may be updated to reflect the physicallocation of the metadata for future reference. The MD data structure 176may be in the form of a lookup table that correlates logical addresses(e.g., LBAs) to the associated metadata units.

Once the data objects and the associated metadata units are stored tothe memory structure 104, read and write processing is carried out toservice access operations requested by a requestor (e.g. host). As shownin FIG. 7, a read request for a selected LBA, or range of LBAs, isserviced by locating the metadata associated with the selected LBA(s)through access to the MD data structure 176 or other data structure, asrepresented at block 180. The physical location at which the metadataunit is stored is identified and a read operation is carried out toretrieve the metadata unit to a local memory (such as a volatile buffermemory of the device 100). The address information for the data objectdescribed by the metadata unit is extracted and used to carry out a readoperation to retrieve a copy of the user data portion of the data objectfor transfer to the requestor, as represented at block 182.

As part of the read operation, the metadata unit may be updated toreflect an increase in the read count for the associated data object,block 184 (if such is stored at the MD level). Other parametricsrelating to the memory may be recorded as well to the memory tier datastructure 162 (FIGS. 5-6), such as observed bit error rate (BER),incremented read counts, measured drift parametrics, etc. It iscontemplated, although not necessarily required, that the new updatedmetadata unit will be maintained in the same memory tier as before.

In the case of rewriteable memory tiers (e.g., tiers 144-150), the newupdates to the metadata (e.g., incremented read count, stateinformation, etc.) may be overwritten onto the existing metadata for theassociated data object. For metadata stored to an erasable memory tier(e.g., flash memory 152), the metadata unit (or a portion thereof) maybe written to a new location in the tier.

It is noted that a given metadata unit may be distributed across thedifferent tiers so that portions requiring frequent updates are storedin one tier that can easily accommodate frequent updates (such as arewriteable tier and/or a tier with greater endurance) and more stableportions of the metadata that are less frequently updated can bemaintained in a different tier (such as an erasable tier and/or a tierwith lower endurance).

FIG. 8 depicts a write operation for a selected LBA, or range of LBAs.

A write command and an associated set of user data are provided from therequestor to the device 100 for storage in the memory structure 104. Asbefore, an initial metadata lookup operation at block 186 locates apreviously stored most current version of the data, if such exists. Ifso, the metadata are retrieved and a preliminary write amplificationfiltering analysis may take place at block 188 to ensure the newlypresented data represent a different version of data such as through theuse of the aforementioned hash values 120 in FIG. 2.

The steps described in FIGS. 5-6 above are repeated for the newlypresented write data. A data object 112 (FIG. 2) is generated and anappropriate memory tier level is selected at block 190. A correspondingmetadata unit 114 (FIG. 2) is generated and an appropriate memory tierlevel is selected at block 192. The data object is stored at block 194,and the metadata unit is stored at block 196. It will be noted that inthe case where a previous version of the data is resident in the memorystructure 104, the new data object and the new metadata unit may, or maynot, be stored in the same respective memory tier levels as the previousversion data object and metadata unit. The previous version data objectand metadata may be marked stale and adjusted as required, such as bythe addition of one or more forward pointers in the old MD unit to pointto the new location, as indicated at block 198.

The metadata granularity is selected based on characteristics of thecorresponding data object. As used herein, granularity generally refersto the unit size of user data described by a given metadata unit; thesmaller the metadata granularity, the smaller the unit size and viceversa. As the metadata granularity decreases, the size of the metadataunit may increase. This is because the metadata needed to describe 1megabyte (MB) of user data as a single unit (large granularity) would besignificantly smaller than the metadata required to individuallydescribe each 16 bytes (or 512 bytes, etc.) of that same 1 MB of userdata (small granularity).

In some cases, the designer may allocate a certain total amount ofstorage for metadata and the rest for user data (data objects). Thequestion then becomes, how best to allocate the metadata for the dataobjects. By adaptively adjusting the granularity of the various metadataunits based on workload, portions of user data that are accessed inlarge sets may justify a smaller metadata granularity, whereas portionsof user data that are accessed in smaller increments may justify anincrease in the metadata granularity for those sets. Once the metadatagranularity is adjusted, a further decision opportunity becomesavailable in that the newly sized metadata may be more effectivelystored in a different tier of the memory structure. Thus, in someembodiments, decisions are made with regard to where data objects andmetadata units are stored in various tiers and then workload and memoryattributes (e.g., endurance, measured performance, aging, etc.) allowboth the data objects and the metadata to be moved to locations thatsupport efficient data I/O rates.

As represented by a metadata granularity adjustment module 200 in FIG.9, the metadata granularity (and metadata level) may be selected, andsubsequently adjusted, based on a number of factors including workload(e.g., read/write counts on the associated data object),throughput/latency of the various memory tiers, endurancecharacteristics of the various memory tiers, and actual or predictedwear of the memory tiers and space available in the memory such as interms of percentage available for each tier or for the total system.

The metadata units are allocated to the various memory tiers accordingto a memory organization scheme that takes into account memoryendurance, speed, power requirements, latency and other memory attributefactors. Data attribute factors include the relative unit sizes of thedata objects (see e.g., FIGS. 4A-4B), the update frequency of the dataobjects, whether the data objects are in a write intensive or readintensive environment, and whether significant read-modify-writeoperations are carried out. Each of these considerations will bediscussed in turn.

With regard to data object size, when a large data object is updated asa unit (see e.g., FIG. 4A), only a relatively small amount of metadatamay be required to describe this information. A relatively smallmetadata unit can be stored in a low bandwidth and low endurance memorytier because the rate of the user data updates in the data object willtend to be relatively low. A small metadata unit can thus be storeddownstream from the data object (e.g., the metadata unit in a lower tierand the data object in a higher tier). A slower readback rate for themetadata unit can be accommodated as compared to the readback rate ofthe corresponding data object since the metadata unit is significantlysmaller than the data object.

Conversely, when a relatively smaller data object (see e.g., FIG. 4B) iswritten frequently with other random small objects in a write intensiveenvironment, the metadata granularity will be adjusted to match thesmall object size. The location of the metadata will be selected suchthat the metadata units for the data objects are in a faster rewriteablememory with greater endurance to accommodate a greater frequency ofmetadata updates.

In a read intensive environment, the granularity of the metadata can beadjusted such that a larger portion, or all, of the metadata can bestored in a memory tier that does not require a large amount of updates.This is because read operations tend to result in relatively smaller (ifany) adjustments to the metadata; no changes in address storage locationfor the data object usually take place unless an error recovery routineis initiated to enable recovery of the data, and so the types ofmetadata updates for read operations may be limited to metrics such asincrements in read counts, observed bit error rates, measured drift,etc.

A read intensive environment may thus enable storage of the metadata ina compressed form to reduce the metadata size. A higher tier of memorycan be used based on the reduced size, although the compressed metadatacan be placed in any suitable location.

In a read-modify-write environment, a relatively large set of user datais transferred to the device from the requestor with updates, butgenerally only a small subset of the data blocks are actually updatedand the rest remain unchanged. For example, a set of 100 LBAs may befrequently presented as a unit for updating, but only a small number ofthe LBAs (e.g., 5-10) are actually modified from one write event to thenext. In such a case, the device may elect to only store the updates tothe changed LBAs and maintain the originally written LBAs in theprevious storage location.

A read-modify-write operation may thus include reading the previouslystored data object, performing a comparison between the previouslystored data object and the new data object to identify those data blockswhich have been identified as having been changed, and writing thechanged data blocks to the memory structure 104. The metadatagranularity may be adjusted to an appropriate level to enable trackingof these changes and allow the device 100 to recover the most currentversion of the data.

In some embodiments, a read-modify-write environment for large dataobjects involving many data blocks (LBAs) may result in the writing ofthe initial data set to an erasable tier (e.g., the flash memory tier152). Updated LBAs may be written to a rewriteable tier (e.g., theNVRAM, RRAM, PCRAM or STRAM tiers 144-150). Once sufficient workloadhistory has been accumulated (e.g., to be able to distinguish betweenread-modify-writes and small random writes), the entire data object maybe migrated to a more suitable memory tier. The granularity of themetadata may be modified and the modified metadata moved to a moresuitable memory tier as well. It can be seen that, generally, the largerthe data objects (e.g., data objects involving many LBAs), the morelikely it may be that read-modify-writes will occur. Adjusting both thedata object and metadata sizes based on workload can thus achieveimproved performance levels that balance metadata complexity andgranularity with storage capacity and throughput.

It is contemplated, although not necessarily required, that each memorytier in the multi-tiered memory structure 104 will store both dataobjects and metadata units (albeit not necessarily related to eachother). It follows that there will be a trade-off in determining howmuch memory capacity in each tier should be allocated for the storage ofdata objects, and how much memory capacity in each tier should beallocated for the storage of metadata. The respective percentages (e.g.,X % for data objects and 100-X % for metadata units) for each memorytier may be adaptively adjusted based on the various factors listedabove. Generally, it has been found that enhanced performance may arisethrough the use of higher memory tiers for the metadata in small randomwrite environments so that the granularity of the metadata can beadjusted to reduce the incidence of read-modify-writes on the dataobjects.

Numerous characteristics and advantages of various embodiments of thepresent disclosure have been set forth in the foregoing description,together with structural and functional details. Nevertheless, thisdetailed description is illustrative only, and changes may be made indetail, especially in matters of structure and arrangements of partswithin the principles of the present disclosure to the full extentindicated by the broad general meaning of the terms in which theappended claims are expressed.

What is claimed is:
 1. An apparatus comprising: a multi-tier memorystructure comprising a plurality of non-volatile memory tiers eachhaving different data transfer attributes and corresponding memory cellconstructions; a data object storage manager adapted to generate firstand second data objects responsive to one or more data blocks suppliedby a requestor and to direct storage of the generated data objects in afirst non-volatile tier of said memory structure, the first and seconddata objects having a common unit size; and a metadata storage manageradapted to generate first and second metadata units, the first metadataunit having a first selected granularity to describe the first dataobject, and to direct storage of the generated first metadata unit in asecond non-volatile tier of said memory structure responsive to thefirst selected granularity, the metadata storage manager adapted togenerate a second metadata unit having a second selected granularity todescribe the second data object, the first and second metadata unitsbeing different, and to direct storage of the second metadata unit inthird non-volatile tier of the memory structure responsive to the secondselected metadata unit, the first, second, and third non-volatile tiersbeing different, the first and second selected granularities eachidentified responsive to an access rate of the respective first andsecond data objects and characterized as a resolution in terms of groupsof bits of user data in the data objects individually described byrespective metadata units.
 2. The apparatus of claim 1, in which thesecond metadata unit is further stored in the second non-volatile tierresponsive to an update frequency of the second metadata unit.
 3. Theapparatus of claim 1, in which each data object comprises at least oneuser data block supplied by a requestor device for storage in themultitiered memory structure, and each metadata object comprises addressinformation associated with a physical address of the respective dataobjects in the first non-volatile tier and state information associatedwith the physical address.
 4. The apparatus of claim 1, in which aselected one of the first or second tiers comprises rewriteablenon-volatile memory and a remaining one of the first or second tierscomprises erasable non-volatile memory.
 5. The apparatus of claim 1, inwhich the multi-tier memory structure provides a plurality of tiers insequential order from a fastest tier to a slowest tier, the second tierbeing slower than the first tier.
 6. The apparatus of claim 1, in whichthe multi-tier memory structure comprises a plurality of non-volatilememory tiers each having different data storage attributes, and theapparatus further comprises selecting the first and second tiers bymatching the data storage attributes of the respective first and seconddata objects to the respective first and second metadata units and therespective first and second tiers.
 7. The apparatus of claim 1, in whichthe first tier has a first data readback rate in terms of continuouslytransferred data bits per unit time and the second tier has a lower,second data readback rate in terms of continuously transferred data bitsper unit time.
 8. The apparatus of claim 1, in which the metadatastorage manager further monitors access operations associated with thefirst data object and adjusts the granularity of the first metadata unitin relation to the monitored access operations.
 9. The apparatus ofclaim 8, in which the metadata storage manager further migrates thefirst metadata unit to the third non-volatile tier of the memorystructure responsive to the adjusted granularity of the first metadataunit.
 10. The apparatus of claim 8, in which at least one of theplurality of nonvolatile memory tiers comprises an array of erasablememory cells and at least one of the plurality of non-volatile memorytiers comprises an array of rewritable memory cells.
 11. The apparatusof claim 1, in which each data object comprises at least one user datablock supplied by a requestor device for storage in the multi-tieredmemory structure, and each metadata object comprises address informationassociated with a physical address of the data object in the firstnon-volatile tier and state information associated with the physicaladdress.
 12. The apparatus of claim 1, wherein a first tier in theplurality of non-volatile memory tiers stores a first plurality of dataobjects and a second plurality of metadata units, wherein none of thesecond plurality of metadata units describe any of the first pluralityof data objects.
 13. The apparatus of claim 1, in which the data objectstorage manager selects the non-volatile first tier for the storage ofthe first data object responsive to an access history of data blocksassociated with the first data object and an attribute of the firsttier.
 14. The apparatus of claim 1, in which the first tier has a firstdata readback rate in terms of continuously transferred data bits perunit time and the second tier has a lower, second data readback rate interms of continuously transferred data bits per the same unit time. 15.The apparatus of claim 1, further comprising a metadata granularityadjustment module which adjusts the granularity of the first metadataunit responsive to an access history associated with the first dataobject and performance attributes associated with the second tier. 16.The apparatus of claim 1, in which the first tier has a slower transferrate performance than the second tier, the data object storage managersubsequently directs storage of a third data object to a different,fourth tier of the multi-tier memory structure, the third data objecthaving the common unit size, the metadata storage manager generates anddirects storage of a third metadata unit to a different, fifth tier ofthe multi-tier memory structure, the third metadata unit having a thirdselected granularity that differs from the first and second selectedgranularities, the fifth tier has a slower transfer rate performancethan the fourth tier.
 17. The apparatus of claim 1, in which themetadata storage manager comprises a metadata granularity adjustmentmodule which adjusts the granularity of at least one metadata unitresponsive to an access history associated with at least one data objectand performance attributes associated with the first memory tier.